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BIG PHYSICS, BIG QUESTIONS –

Ecosystems still feel the pain of ancient extinctions

By Michael Marshall

IT’S not just humans that still reel from the effects of a trauma many years later&colon; ecosystems do too. Thousands of years after human hunters wiped out big land animals like giant ground sloths, the ecosystems they lived in are still experiencing the effects.

Many ecosystems rely on big herbivores to spread nutrients, mostly in the form of dung. “If you remove the big animals from an ecosystem, you pretty much stop nutrients moving,” says Chris Doughty at the University of Oxford.

Doughty and colleagues simulated the distribution of phosphorus, a nutrient that plants need to grow, in the Amazon basin in South America. This area was once home to spectacularly large herbivores, such as the elephant-like gomphotheres and giant ground sloths (pictured).

But about 12,500 years ago, around the time humans moved into South America, these huge animals all died out, hit by the double whammy of hunting and a changing climate. Nowadays the Amazon is home to a huge diversity of animals. “But these extinctions cut out all the big animals,” says Doughty.

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It seems the mass extinction had a profound effect on how phosphorus is spread around the Amazon basin. Nutrients are released when rocks are eroded, and then get distributed onto flood plains by rivers. In South America, the most phosphorus-rich soils – and hence plants – are found near the Andes mountain chain in the west, and alongside the rivers flowing from it, especially the Amazon. Animals eat these phosphorus-rich plants, then move on to a new area and release the phosphorus when they defecate.

Using the relationship between living animals’ ability to spread nutrients and their size as a guide, Doughty estimated how much phosphorus South America’s larger extinct animals would have moved 15,000 years ago, before their decline.

The model suggests that megafauna would have spread nutrients 50 times further than animals today do over the same time period. Or to put it another way, killing off the massive animals reduced the movement of nutrients by 98 per cent. This is because big animals move a disproportionately large amount of nutrients compared with small animals, they travel further in search of food, and they keep that food in their guts for longer (Nature Geoscience, doi.org/nfm).

Doughty compares big animals to the arteries that carry blood around the body. “When you get rid of big animals, it’s like severing the nutrient arteries.” He thinks the same thing has happened in North America, Europe and Australia, where most big animals have also been wiped out.

When you get rid of the biggest animals, it’s like severing the ecosystem’s nutrient arteries

“The idea that herbivores redistribute nutrients is not new, but the scale of this thinking is much, much bigger,” says Tim Baker at the University of Leeds in the UK.

If Doughty is right, the Amazon is still changing in response to the extinction. His model predicts that nutrient distribution will get patchier for another 17,000 years, although the effect will probably be dwarfed by the impacts of deforestation and climate change in the short term, says Baker.

In the absence of massive herbivores, humans now dominate the movement of nutrients – but we do the opposite of what the extinct animals did. We spread fertiliser on small plots of productive farmland, and keep large animals like cows fenced in rather than letting them roam freely. “There are probably more nutrients because of people, but they are very poorly distributed,” says Doughty.